10 System and performance requirements
11.2 Performance in LOS channels
11.2.1 Forward link PER performance
Physical layer performance has been carried out considering the complete DVB-S2 transmit-receive chain in a railway environment. LOS channel condition (Rice factor K = 17,4 dB) is assumed. Two different train speeds have been considered: 30 km/h and 300 km/h. Simulations have been carried out under the hypothesis of a receiving antenna with high directivity. In other words, the Doppler spectrum is reduced with respect to the Jakes model to take into account the decreased multi-path power captured by the receiving antenna (see clause11.1.1). The carrier phase noise is generated according to the phase noise mask reported in the DVB-S2 implementation guidelines [i.29] and the non-linear distortion introduced by the HPA has been kept into consideration according to the data reported in clause 5.1.1.1. The predistortion technique applied in the following is based on a fractional approach (post matched filter) described in [i.30].
Table 18: HPA related parameters Modulation IBO (dB) OBO (dB)
QPSK 0,5 0,33
8-PSK 1 0,43
16-APSK 2 1,08
For the performance analysis purposes, the digital receiver architecture depicted in figure 51 has been considered. It is worthwhile noting that since this analysis is focused on the data detection performance of the receiver, the initial frequency acquisition and frame synchronization operations are assumed to be successfully accomplished, (see clause 5.1). Hence, the white block of figure 51 has not been considered in the software simulations for this analysis.
In the following, a brief description of the different sub-subsystems is reported in order to ease the understanding of the performance results reported in this clause. The carrier frequency coarse correction is firstly performed to allow matched filtering with minimal inter-symbol interference re-growth; then the clock recovery for timing adjustment is performed by a digital interpolator. A demultiplexer is used to separate pilots from data symbols carried in the PLFRAME. The pilot symbol stream is used by the following four sub-systems: the noise level estimator, the digital Automatic Gain and Angle Control (AGAC), the block in charge of tracking the residual frequency offset and carrier phase and, finally, the coarse frequency acquisition loop (not performed in hot start). On the other path, the data symbols feed the hard/soft demodulator. The demodulator provides the hard decisions on data symbols as a feed-back for carrier frequency and phase tracking, and computes the soft initial A Posteriori Probability (APP) on the received information bits. Finally, the APPs are de-interleaved and provided to the LDPC-BCH decoder. From the point of view of the sequential order of operations, during initial acquisition, the first operating sub-system is the clock recovery;
since it can operate in the presence of large carrier frequency errors. For example, the Gardner timing detector [i.31]
exhibits good estimation performance also in the presence of rather high-carrier frequency mismatch. Then, after the frame synchronization algorithm, the pilot symbols can be used to initiate the frequency recovery loop. Upon reaching the steady state, the fine channel tracking operations is run. In particular, the adopted noise level estimation algorithm is derived from the maximum likelihood (ML) theory [i.32]. The data-aided (DA) version is considered, since the
estimator operates on the PLHEADER (90 known symbols: 26 from the SOF field and 64 from the MODCOD
information). The digital AGAC sub-system [i.33] is a feed-forward algorithm based on an ML approach which exploits the presence of the pilot symbols by estimate the channel coefficients as follows:
∑
∑
−
=
−
=
= 10 2 1
0 ) (
*
ˆ
Pi i P
i
p i i
k
d r d c
where P is the number of pilot symbols, di is the known pilot symbol, and finally
) ( p
ri
is the received pilot symbol.
Each fading estimate is then linearly interpolated between consecutive pilot fields to better tracking the channel propagation fluctuations. This solution is supported by the fact that the channel coherence time is always longer (90 μs;
at least) than the time distance between two consecutive pilot slots (50 μs at 27,5 Msps). Finally, the AGAC angle estimate initializes the block in charge of tracking the residual frequency offset and carrier phase-noise fluctuations. For this purpose, a well-known second-order loop filter is implemented.
Frequency Acquisition
Matched Filter
Symbol Sampling
Timing Recovery
DeMUX
Preamble / Pilots
Digital AGAC
Hard/Soft Demodulator Data
Freq/Phase Tracking
LDPC/BCH Decoder Noise level
Estimation Buffer
Buffer
De-Interleaver Lock
Detector Frame
Synch
ˆ0
θ
ˆ0
θ
θˆk
ˆ0
N
ak
1 ( )2
Figure 51: Block diagram of the digital receiver
Figure 52 and figure 53 report packet error performance measured on the DVB-S2 DATAFIELD (see figure 3) without spreading, for 30 km/h and 300 km/h, respectively, confirming that in LOS conditions, the detection performance is still satisfactory for both high and low train speed.
1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Es/N0 [dB]
PER
1/4 QPSK - LOS, 30 km/h 1/3 QPSK - LOS, 30 km/h 1/2 QPSK - LOS, 30 km/h 3/4 QPSK - LOS, 30 km/h 3/5 8PSK - LOS, 30 km/h 3/4 8PSK - LOS, 30 km/h 2/3 16APSK - LOS, 30 km/h 3/4 16APSK - LOS, 30 km/h
Figure 52: DVB-S2 performance for several MODCODs, in LOS channel condition, Ku band, no spreading, low train speed
1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Es/N0 [dB]
PER
1/4 QPSK - LOS, 300 km/h 1/3 QPSK - LOS, 300 km/h 1/2 QPSK - LOS, 300 km/h 3/4 QPSK - LOS, 300 km/h 3/5 8PSK - LOS, 300 km/h 3/4 8PSK - LOS, 300 km/h 2/3 16APSK - LOS, 300 km/h 3/4 16APSK - LOS, 300 km/h
Figure 53: DVB-S2 performance for several MODCODs, in LOS channel condition, Ku band, no spreading, with and high train speed
11.2.2 Forward link spectrum spreading performances
In the following, the impact on performance of the introduction of spread spectrum techniques is presented in terms of BER and PER. In particular, the following system parameters have been considered:
• MODCOD: 1/4-QPSK.
• Chip rate = 27,5Mchip/sec.
• Symbol rate = Chip rate / Spreading factor.
• Train speed = 300 km/h.
• Propagation channel: AWGN and correlated ricean channel with Rice factor = 17 dB.
• Satellite HPA IBO = 0,5 dB.
• No interference from adjacent satellites.
First of all, the robustness of the DVB-S2 spread signal with respect to non-linear distortion is presented in figure 54 with AWGN channel. The comparison between the spread and non-spread signals highlights that the signal spreading slightly increases the robustness to non-linear distortion.
Spreading vs No Spreading with non linear HPA
1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Eb/N0 [dB]
BER
QPSK 1/4 IBO=2 - SPREADING FACTOR=2 QPSK 1/4 IBO=2 - NOSPREADING QPSK 1/4 IBO=0.5 - SPREADING FACTOR=2 QPSK 1/4 IBO=0.5 - NOSPREADING
Figure 54: Comparison between spread and not spread signal in AWGN channel with the presence of non-linear HPA
The terminal mobility impact on spread waveform performance is analysed in figure 55. The same chip-rate has been considered, thus the transmission symbol rate is 13,75 Msps and 6,85 Msps for SF = 2 and SF = 4, respectively. As noted before, spectrum spreading introduces a slight gain with respect to the unspread signal. The real benefit of the introduction of the spreading factor is in term of link budget, showing a gain of 3 dB and 6 dB for spreading factor 2 and 4, respectively.
1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00
-3 -2 -1 0 1 2 3 4 5 6 7 8
Es/N0 [dB]
PER
1/4 QPSK - LOS, 300 km/h 1/4 QPSK - SF=2 1/4 QPSK - SF=4
Figure 55: Comparison between spread and not spread signal in LoS channel with the presence of non-linear HPA